Chapter 1 Status and Trends of Global Biodiversity


Biodiversity of Marine and Coastal Ecosystems

The major zones of the oceans

The area of the ocean most influenced by human activity, and best known scientifically, is the continental shelf - the area of shallow water lying around the major landmasses that may be anything from a few kilometres to several hundred kilometres wide. The most landward part of this is the intertidal or littoral zone, where the bottom is periodically exposed to air and water depth varies from zero to several metres. Seaward of this the shelf slopes gently from shore to depths of one hundred to several hundred metres, forming the sublittoral or shelf zone. At the outer edge of the shelf there is an abrupt steepening of the sea bottom, forming the continental slope, which descends to depths of 3-5 km. At this level there are immense abyssal plains, which form the floor of much of the world's oceans. The plains are punctuated by numerous submarine ridges and seamounts, which may break the sea-surface to form islands. The deepest parts of the ocean are ocean trenches, which are seismically highly active, and reach depths of from 7000 to 11,000 metres.

In the sea, as on land, photosynthesis based on sunlight is the driving force behind the maintenance of life. Because seawater absorbs sunlight strongly, photosynthesis is limited to the topmost layers of the sea, the so-called euphotic zone, which rarely reaches depths of more than 200 m in the open ocean, although at depths between 200 m and one kilometre blue light may still penetrate sufficiently to allow limited photosynthesis. Primary producers, largely in the form of photosynthesising bacteria and algae, are effectively confined to this zone. The few exceptions include bacteria living around hydrothermal vents associated with rift zones in the ocean floor. These bacteria use hydrogen sulphide as an energy source and support a unique community of other organisms. They are scientifically of enormous interest, but make a negligible contribution to overall productivity in the oceans. Areas of high productivity are associated with upwelling zones, where nutrient-rich bottom water is brought to the surface.

Major marine communities

Mangroves or mangals are truly hybrid terrestrial/marine ecosystems. They are a diverse collection of shrubs and trees (including ferns and palms) which live in or adjacent to the intertidal zone and are thus unusual amongst vascular plants in that they are adapted to having their roots at least periodically submerged in sea water. Mangrove communities are predominantly tropical and are rarely found beyond the latitudes 30_N and 30_S. They are only able to grow on shores that are sheltered from wave action. Diversity in mangrove ecosystems is usually relatively low, particularly when compared with other tropical ecosystems such as tropical moist forests and coral reefs. The most diverse mangrove systems are found in Southeast Asia (Map 12).

Coral reefs are calcium carbonate structures developed very largely by stony corals in the order Scleractinia of the phylum Cnidaria. They are essentially tropical, shallow water ecosystems largely restricted to the area between latitudes 30_N and 30_S and most abundant in shallow, well flushed marine environments characterised by clear, warm, low-nutrient waters that are of average oceanic salinity (Map 6). It is difficult to measure the global area of coral reefs, although it is believed to exceed 600,000 km2. Near-surface reefs (the most productive and diverse) are estimated to cover around 255,000 km{They are among the most productive and diverse of all natural ecosystems, although it appears that their diversity is generally lower than that of the tropical moist forests often regarded as their terrestrial analogues. As with mangroves, the centre of diversity of reef-building corals is Southeast Asia, with an estimated minimum of 450 species of corals found associated with reefs around the Philippines, Borneo, Sulawesi and associated islands.

Map 6

Seagrasses are flowering plants (not true grasses) that are adapted to live submerged in seawater. There are approximately 48 species found in shallow coastal areas between the Arctic and Antarctic, down to depths of around 60 m. The most extensive areas are on soft substrates. Seagrass beds have high productivity and contribute significantly to the total primary production of inshore waters, being an important source of food for many organisms. The total global area is unknown.

Algae are the major large photosynthesising organisms in marine environments. There are three main groups: the red algae or Rhodophyta with around 4000 marine species, the green algae or Chlorophyta with around 1000 species, and the brown algae or Phaeophyta with about 1500. In many of the colder regions of the world, hard subtidal substrates are occupied by very large brown algae collectively known as kelps (order Laminariales), which form extensive kelp beds or forests. The cooler regions of the world are rich in algal species.

Pelagic or open-ocean communities occupy a greater area than any other major community type on earth. They are dominated by the activity of plankton in the euphotic zone near the surface. The plankton support a large number of free-swimming organisms or nekton, predominantly fishes but also cetaceans and cephalopod molluscs (squid). The marked vertical gradients within the pelagic zone - of light, temperature, pressure, nutrient availability and salinity - lead to strong vertical structuring of pelagic species assemblages. This vertical stratification is not static, but fluctuates seasonally and, often, daily, with variations in the physical and chemical characteristics of the seawater and vertical migrations of mid-water organisms. Until recently it was assumed that biomass in the pelagic zone everywhere below the euphotic zone was low. Recent studies have indicated that in some areas (for example the northern Indian Ocean), biomass of tropical mid-water or mesopelagic animals may be surprisingly high.

Deep-sea communities are prevalent on around half of the world's surface area, where the ocean is over 3000 m deep. All such areas are in permanent darkness (other than light generated by bioluminescence and tectonic activity). Biomass is very low but new sampling techniques indicate that the diversity of small organisms in or on the sea-bottom is often relatively high.

Major values and uses of marine and coastal ecosystems

The world ocean plays a crucial role in the regulation of climate and in the carbon cycle and other biogeochemical cycles. Much of this role can be ascribed to the ocean as a physical entity, that is as a vast, circulating body of water whose high specific heat capacity buffers atmospheric changes in temperature and which is capable of dissolving very large amounts of carbon dioxide. It does seem however that marine organisms also play a very important role in the major biogeochemical cycles, although one which is difficult to quantify.

Marine fisheries provide by far the largest source of wild protein, this being of critical importance to many subsistence communities around the world, making use of fin fishes, crustaceans and molluscs, with marine algae increasing in importance in Asia. Mangroves and seagrass beds fulfil an important function as nursery areas for juvenile fish and shellfish.

Marine capture fisheries have increased in volume nearly five-fold in the past 50 years, rising to nearly 90 million tonnes in the late 1990s, this making up more than 70% of the total world production of aquatic resources (Map 7). Analysis by FAO and others indicates that marine stocks are widely overexploited and in urgent need of remedial management. Other major uses of the marine sphere include:

  • waste disposal;
  • recreation, chiefly in coastal zones;
  • coastal stabilisation;
  • transportation.

Major impacts on marine and coastal ecosystems

Impacts can be grouped into five main categories:

  • chemical pollution and eutrophication;
  • fisheries operations;
  • global climate change;
  • alterations of physical habitat;
  • invasions of exotic species.

Map 7

Chemical pollution with eutrophication is a widespread problem, most pronounced in semi-enclosed seas such as the Mediterranean and the Black Sea.

In addition to the potential for over-fishing, fishery operations can have a destructive physical impact on the seabed, and affect population levels of non-target species through incidental catch, such problems being of particular significance for cetaceans, sea turtles and seabirds such as albatross, in different parts of the world. All commercial bottom fishing disturbs sea-floor organisms and the seabed, with impacts on both habitats and species.

Coral-bleaching related to periodic climatic events is the primary threat to coral reefs on the global scale. The increase in sea-surface temperature associated with the major El Niño and La Niña climate switches in 1997-1998 resulted in extensive coral bleaching and mortality over large portions of the Indian Ocean and Southeast and East Asia. On some reefs, there were mortality levels greater than 90% leaving some reefs almost bare of corals and with early indication of major shifts in the population structures.

The critical feature of recent coral-bleaching events are that areas have been struck indiscriminately, irrespective of the status of reef health; impacts have been felt both on pristine, remote reefs and on reefs already under major human-induced stresses. Some of the changes recently caused by periodic climate events, including coral bleaching are not necessarily permanent, however human-induced stresses causing physical degradation and destruction to coral-reef organisms often exacerbate the effects these events or limit the recovery capability of reef ecosystems.

Status and trends of species in marine and coastal ecosystems

As a gross generalisation, marine species appear to be somewhat less prone to extinction than inland water or terrestrial ones. Certainly, available information indicates that far few marine species are known to have become extinct since 1600 than either terrestrial or freshwater ones. Part of this difference may be because marine species are in general much less readily observable than those in terrestrial habitats, and, to a lesser extent, inland water species. It is possible therefore that a higher proportion of marine than terrestrial or inland water species has become extinct without our knowing. However, it seems likely that the difference is in large part a real one, and can be ascribed to two main factors. First, because of the size of the oceans, and the fact that people do not live permanently in them, they are as a whole considerably more buffered from human impacts than terrestrial or inland water areas. Second, marine species appear on the whole to be more widespread than terrestrial or inland water ones. This makes them generally less vulnerable to extinction. There are, of course, many exceptions to this.

Notes on a selection of the marine species that are categorised as Critically Endangered in the 2000 IUCN Red List, i.e. at highest risk of extinction, are provided in Table 1.5 below.

Table 1.5 Some Critically Endangered marine species
Phocoena sinus
  A small porpoise restricted to the Gulf of California, Mexico. The vaquita is threatened by accidental entanglement and drowning in fishing nets.
Mediterranean Monk Seal
Monachus monachus
  Formerly widespread throughout the Mediterranean and North African coast; now mostly restricted to islands in the Aegean Sea and the coast of Mauritania. Threatened by entanglement in fishing nets, disturbance of breeding sites and illegal killing.
Amsterdam Albatross Diomedea amsterdamensis   A large seabird nesting only on Amsterdam Island in the Southern Indian Ocean. The small population (approximately 70 individuals in 1990) was previously threatened by habitat loss, and is at risk from entanglement in fishing gear.
Fiji Petrel
Pseudobulweria macgillivrayi
  Until 1984 known from a single specimen collected in 1855 from Gau Island, Fiji. Nesting is believed to occur in mature forest on the island, though nests have yet to be located. Feral cats are a potential threat to this little-known seabird.
Galápagos Petrel
Pterodroma phaeopygia
  Breeds only in the highlands of the Galápagos Islands, Ecuador. Destruction of nest sites and predation by introduced animals has resulted in a steady population decline.
Kemp's Ridley
Lepidochelys kempii
  Mainly occurs in the waters of the western Atlantic and nests almost exclusively at a single beach in Mexico. Predation of eggs, illegal catch of adults and entanglement in fishing gear has led to the species decline. The species is now recovering through intensive conservation measures.
Spotted Handfish Brachionichthys hirsutus   Endemic to the Derwent estuary, Tasmania, Australia. Causes of decline not clear but predation of handfish eggs by an introduced starfish species and increases in soil runoff from surrounding agricultural land may be the principal factors.
Totoaba macdonaldi
  A large fish restricted to the Gulf of California, Mexico. Threatened by excess fishing, mortality of juveniles in shrimp fishery bycatch and possibly by a reduction of freshwater discharge into the Gulf.
Largetooth Sawfish
Pristis perotteti
  These large unusual fish occur mainly in the shallow estuarine waters of the Atlantic, eastern Pacific and possibly the Mediterranean. Decline of this slow maturing species attributed to excess fishing.

  • United Nations
  • United Nations Environment Programme